Many tennis players try many different techniques to generate spin when they hit the tennis balls. With an ultra-high-speed, 10,000-frame-per-second camera, we can now clearly see how strings generate spin during the 4 or 5 milliseconds when the ball is on the strings. Most spin is generated by main strings (vertical strings) slide and snap back movement after they contact with the tennis ball. Thus, the rougher the main string surface is, the more main string slide and snap back with more power for spin generation, and the easier it is for the player to generate spin. Basically, in order to generate more spin, we need two factors: 1. a rough string/ball contact surface, therefore the strings can grasp the ball better, and the ball can push the main strings to slide more over the cross strings(cross strings); as a result, the main strings can generate more spin when they snap back. 2. a smooth main string/cross string contact surface, therefore the main strings can slide farther over the cross strings and snap back efficiently because of less frictions between the strings, this will give an extra kick to the ball and generate more spin. In order to satisfy two factors stated above, you need a main string that has two different surfaces, rough on the top (where it contacts with the ball) and smooth on the bottom (where it contacts with the cross string) in order to generate spin efficiently.
Most strings are made of synthetic nylon and polyester, and some of them are made of natural gut. Some string manufacturers provide Octagon or Hexagon shaped polyester strings to increase the friction between the string and ball. It may be effective on the main strings when they are new, after some time, the sharp edge of the Octagon and Hexagon will be worn away by wear and tear between the string and ball. As a result, the main strings can no longer grasp and generate spin as effective as when they are new. Some strings come with a series of small dents on the surface to increase the roughness of the string surface. This is effective when it is new, however, after some time, the small dents will be worn off and the string surface becomes smooth again.